design considerations for aluminum hull structures - Ship Structure ...

-112-fatigue, particularly in the presence of salt spray, as well as fracturetoughness and abrasion resistance relative to mild steel. Limited testdata indicates a significant reduction in the endurance limit of aluminumalloys when subjected to salt spray.3. The area under the S-N fatigue curve of welded higher strengthS000 series alloys (5083 or S4!56)is about 0.48 times that of mild steel,while the corresponding value for lower strength S086 alloy is about 0.38.This is indicative of the relative required section moduli of the hullgirders of an aluminum and steel bulk carrier for equivalent fatigue life..4* The notch and fracture toughness of aluminum alloys appearacceptable for hull structural applications. However, stress levels,including effects of stress concentrations, should be kept below theyield stress.5. The corrosion resistance of SOOO series aluminum alloys isacceptable for a marine environment if proper precautions are taken. Therecent introduction of the IH16 and 1311’7temper has apparently solved theexfoliation problem, and suitable gasketing and isolation procedures areavailable to minimize problems with dissimilar metals. Higher magnesiumalloys are somewhak susceptible to stress corrosion problems which mustbe considered in selection of tempers and operating temperatures. Lossof strength and thickness of aluminum alloys in a salt water environmentover a 20 year vessel life will riotbe significant.6. The corrosion resistance of S000 series alloys is acceptable forthe range of bulk cargoes and liquids which might be carried, with theexception of copper, tin or mercury ores, potassium hydroxide and carbonateand trisodium phosphate. Precautions are required for a limited number ofother potential cargoes.7* The abrasion resistance of aluminum when subjected to the loadingand unloading of bulk cargoes will be significantly less than that of sbeel,necessitating additional margins in tank top and lower bulkhead thicknesses.8. The weldability and workability of ~000 series aluminum alloysare very good, and the state-of-the-art in welding technology is presentlyadequate for the thicknesses of material being considered. Potentialproblem areas such as control of shrinkage, weld sequence, structuraldetails, residual stresses ad environmental protection require furtherStudy.7. The cost of the ~000 series alloys being considered is relativelyindependent of alloy and temper, and has little effect upon the selectionof alloys.10. Alloy S083 was selected for all material in the primary hullstructure of the bulk carrier, based upon its high strength and goodworkability. Alloy 5086 may be subskihuked for S083 for seconda~ structures,and alloy s~~sbis ‘Lobe used in areas of high temperature. Therelative over-all ranking of the JOOO series alloys considered was veryclose, so thak alternative selecti-ons can be juskified.11. Operationswith Existing Aluminum Ships - Experience to date withlarge aluminum ships is limited, but considerable data is available on theperformance of aluminum deckhouses, patrol craft, crew boats and pleasurecraft. This data indicates that the performance of recently-built aluminum